1. Field of the Invention
This invention relates to a sheet guide apparatus which is provided, for example, in a printing press for printing on a sheet, or a coating device for applying a coating onto a sheet, and which is adapted to transport the sheet in a stable state.
2. Description of the Related Art
In a multicolor (e.g., four-color) sheet-fed printing press, as shown in FIG. 13, transfer cylinders (intermediate cylinders) 104 are arranged between printing units 103a to 103d in a printing section 102 located between a feeder 100 and a delivery unit 101. A printed sheet is moved from an impression cylinder 105 of the preceding printing unit to the transfer cylinder 104, and then to an impression cylinder 105 of the succeeding printing unit via gripper devices (sheet gripping devices; not shown). The printing press for thin sheets employs the transfer cylinder 104 of a cylindrical shape. Whereas the printing press for thick sheets employs a skeleton cylinder so that the thick sheet having nerve will not be warped greatly.
In recent years, the printing press for both of thin sheets and thick sheets has been demanded, and the printing press using skeleton cylinders and suitable for thin sheets and thick sheets has appeared. The weakness of this printing press is in handling thin sheets, as will be understood from the above description. A thin sheet without nerve is not supported by the cylinder, and thus makes an unstable motion, thereby causing a printing trouble. This is true of the coating machine for applying a coating onto a sheet. If the sheet is thin, a coating failure occurs. Conventionally, therefore, various sheet guide devices (see sheet guide devices 106 in FIG. 13) have been provided along transfer cylinders, constructed as skeleton cylinders, for transferring a sheet to impression cylinders, in order to stabilize the sheet being transported, thereby preventing a printing trouble or a coating failure.
Japanese Unexamined Patent Publication No. 2001-293843, for example, discloses the following technique: Air within an air chamber, provided at a back site of a sheet guide (see the sheet guide device 106 in FIG. 13) downstream in the direction of sheet transport, is ejected in a direction along the direction of rotation of an impression cylinder, as a transport cylinder, to generate a negative pressure on the lower surface of a sheet passing over an end portion of the sheet guide surface, thereby imparting moderate tension to the sheet and stabilizing the behavior of the sheet on the impression cylinder as the transport cylinder. Also, air near the position of transfer of the sheet is sucked to avoid scattering of air and make the sheet transferred smoothly from the intermediate cylinder to the impression cylinder.
Japanese Unexamined Patent Publication No. 2001-293844 discloses the feature that suction holes are provided in a sheet guide surface on a downstream side of a sheet guide (see the sheet guide device 106 in FIG. 13), and air near the front end of the sheet guide is sucked through these suction holes and suction holes provided in the wall surface of a suction chamber of the sheet guide, whereby fluttering of an end portion of the sheet or instability of the sheet is suppressed.
The foregoing earlier technologies, however, are unable to stabilize the sheet transported in a region which is present between the front end of the guide member and the position of transfer and where the sheet is not guided by the guide member. That is, according to Japanese Unexamined Patent Publication No. 2001-293843, a negative pressure generated by ejection of air is so weak that this negative pressure exerts an extremely low effect on the above-mentioned region. Moreover, the suction holes are remote from this region, so that a negative pressure due to suction through these suction holes exerts an extremely low effect on the region. Thus, the sheet cannot be stabilized in this region, causing a printing trouble.
According to Japanese Unexamined Patent Publication No. 2001-293844 as well, the suction holes are remote from the above-mentioned region, so that a negative pressure due to suction through these suction holes exerts an extremely low effect on the region. Thus, the sheet cannot be stabilized in this region, causing a printing trouble. Furthermore, a suction force ascribed to the suction holes provided in the sheet guide surface on the downstream side of the sheet guide is difficult to adjust. A weak suction force cannot suppress instability of the sheet. A strong suction force, on the other hand, causes the sheet to move while keeping hard contact with the guide surface, causing scratches or cracks to the sheet.
Besides, the printed sheet is transferred from the impression cylinder to the transfer cylinder, and it has been found that after gripping change to the transfer cylinder, the printed sheet gradually increases in instability, seriously affecting printing quality. To suppress this sheet instability, air may be blown through a discharge nozzle 107 from above an impression cylinder 105, as shown in FIG. 12, to suppress sheet instability. However, if air is blown onto the trailing edge of a printed sheet W, sheet instability is likely to occur on the trailing edge of the printed sheet W.